1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer, for visualizing an electrostatic latent image formed on an image bearing member with toner to obtain an image.
In particular, the present invention relates to an image forming apparatus in which a high-capacitance photoconductive layer whose capacitance per unit area is equal to or larger than 1.7×10−6 (F/m2) is used for the image bearing member.
2. Description of the Related Art
In general, in an electrophotographic image forming apparatus, an organic photoreceptor is widely used as a photosensitive member which is an image bearing member. FIG. 5 is a layer structural model view showing an example of the organic photoreceptor. In the organic photoreceptor, a charge generation layer 12 made of an organic material, a charge transport layer 13, and a surface protecting layer 14 are stacked on a metal base 11 in this order. A photoconductive layer of the organic photoreceptor includes the charge generation layer 12, the charge transport layer 13, and the surface protecting layer 14 except the metal base 11. Also, even if an undercoat layer is applied on the metal base 11, the photoconductive layer means the charge generation layer 12, the charge transport layer 13, and the surface protecting layer 14, that is, except for the metal base 11 and the undercoat layer (not shown). Incidentally, for the photosensitive member having no surface protecting layer 14, the photoconductive layer means the charge generation layer 12 and the charge transport layer 13.
A copying machine or a printer which is the electrophotographic image forming apparatus is greatly expected to move into a quick printing market with the advance of digitization, full colorization, and increase in speed on the copying machine or the printer.
However, in order to move into the quick printing market, it is essential to use an electrophotographic image forming apparatus capable of forming an electrostatic latent image with high resolution. In the case of the organic photoreceptor having a charge generation mechanism in an inner portion of the organic photoreceptor, charge diffusion occurs before charges reach a surface of the organic photoreceptor, so that there is a limit to an increase in resolution.
In recent years, a method using an amorphous silicon photosensitive member having a mechanism in which charges are generated in the vicinity of a surface of the amorphous silicon photosensitive member has drawn attention as a method of minimizing the charge diffusion to realize the increase in resolution (see, for example, Japanese Patent Application Laid-open No. 2004-279902).
In the amorphous silicon photosensitive member, as shown in a layer structural model view of FIG. 6, an amorphous silicon photoconductive layer is fundamentally formed on the metal base 11. The amorphous silicon photosensitive member is fundamentally united by superposing, on the metal base 11, the charge blocking layer 15, the charge generation layer 16, the charge blocking layer 17, and the surface layer 18 in order of mention as shown in the layer structural model view of FIG. 6. Here, the photoconductive layer of the amorphous silicon photosensitive member means the remaining obtained by eliminating the metal base 11 from the photosensitive member. That is, the photoconductive layer of the amorphous silicon photosensitive member means the charge blocking layer 15, the charge generation layer 16, the charge blocking layer 17, and the surface layer 18. The amorphous silicon photoconductive layer 15 is made of an amorphous silicon material such as a-Si, a-SiC, a-SiO, or a-SiON and formed by, for example, a glow discharge decomposition method, a sputtering method, an electron cyclotron resonance (ECR) method, or a vapor deposition method.
In order to obtain a high-resolution image, it is desirable that the photoconductive layer of the image bearing member have a high capacitance per unit area. To be specific, it is desirable to use a high-capacitance photoconductive layer whose capacitance per unit area is equal to or larger than 1.7×10−6 (F/m2). The amorphous silicon photosensitive member (hereinafter referred to as a-Si photosensitive member) has a permittivity approximately three times that of the organic photoreceptor. Therefore, when a thickness of the amorphous silicon photosensitive member is equal to a thickness of the organic photoreceptor (each of which is a total thickness obtained by subtracting a thickness of the metal base serving as a support member from a thickness of the photosensitive member), the capacitance per unit area of the photoconductive layer of the amorphous silicon photosensitive member becomes approximately three times that of the organic photoreceptor. In order to improve dot reproducibility, it is necessary that the thickness of the photoconductive layer be thinned to suppress the charge diffusion. In the case of the a-Si photosensitive member, it is found that it is necessary to set the thickness of the photoconductive layer to 50 μm or less to realize allowable dot reproducibility. In the case of the organic photoreceptor, it is found that it is necessary to set the thickness of the photoconductive layer to 17 μm or less to realize dot reproducibility equal to that of the a-Si photosensitive member. In this case, the capacitance per unit area of the photoconductive layer of the image bearing member is set to a lower limit. For the above-mentioned technical reasons, the above-mentioned capacitance (≧1.7×10−6 (F/m2)) is set.
Incidentally, the capacitance of the photoconductive layer is a value measured by the following method. A flat-shaped photosensitive plate is prepared by forming a layered structure same as the actual photoconductive layer on a metal base. An electrode smaller than the flat-shaped photosensitive plate is contacted to the flat-shaped photosensitive plate. A direct current voltage is applied to the electrode. At this time, the current passing the electrode is monitored. The monitored current is integrated in time to obtain a charge amount q accumulated in the photoconductive layer. These steps are performed while changing a value of the direct current. The capacitance of the photosensitive plate is obtained from an amount of change in the charge amount q. In this embodiment, the measurement is performed by using the flat-shaped photosensitive plate. If, however, the shape of an electrode is contrived so as to have the same curvature as the photosensitive member, the measurement can be performed by using a drum-shaped photosensitive member.
When the amorphous silicon photosensitive member (hereinafter referred to as a-Si photosensitive member) having the high capacitance is used, an extremely high-resolution image can be outputted. However, an image defect “blank area” generates unlike the case where the organic photoreceptor having the low capacitance is used.
As shown in FIG. 7, the “blank area” is an image defect in which, when an image pattern in which a medium contrast portion “A” and a high-density portion “B” are adjacent to each other in a traveling direction (i.e., rotating direction) of a surface of the a-Si photosensitive member is outputted, a density of an interface portion “C” there between significantly reduces. With respect to the traveling direction of the surface of the a-Si photosensitive member, there are both the case where the high-density portion “B” is located prior to the medium contrast portion “A” and the case where the medium contrast portion “A” is located prior to the high-density portion “B”.
As a result of concentrated studies on the generation reason of the “blank area”, it is found that the “blank area” is generated by the following mechanism. This will be described in detail below.
FIG. 8 shows an electrostatic latent image potential on a photosensitive member. This example shows a combination of a photosensitive member-negative charging processing, image exposure, and reversal development. An unexposed portion becomes a background portion (i.e., non-image portion). In FIG. 8, reference symbol Vl denotes a latent image potential of the high-density portion “B” on an image area, reference symbol Vh denotes a latent image potential of the medium contrast portion “A” on the image area, and reference symbol Vd denotes a latent image potential of a non-image portion D. When a development bias voltage is applied to a developing sleeve serving as a developer carrying member of a developing device in order to perform development with the latent potentials, toner is transferred from the developing sleeve to the image area of the photosensitive member to develop a latent image. This is because each of a development contrast voltage Vcont corresponding to a potential difference between a direct current voltage component Vdc of the development bias voltage and the latent image potential Vl of the high-density portion “B” on the image area, and a development contrast voltage Vcont-h corresponding to a potential difference between the direct current voltage component Vdc of the development bias voltage and the latent image potential Vh of the medium contrast portion “A” on the image area, is to be filled with (i.e., to be eliminated by) toner charges.
However, there is a difference between the development contrast voltages Vcont and Vcont-h in the interface portion between the medium contrast portion “A” and the high-density portion “B”, so a wraparound electric field extending from the medium contrast portion to the high-density portion is formed near the surface of the photosensitive member in addition to an electric field extending from the developing sleeve to the photosensitive member. FIG. 10 shows electric field vectors located near the surface of the photosensitive member in the interface portion between the high-density portion and the medium contrast portion. In the beginning of a developing process, toner present near the interface portion and toner present in the medium contrast portion follow the electric field vectors shown in FIG. 10 by the wraparound electric field, so that most of the toners transfers to a latent image of the high-density portion.
When the development contrast voltage Vcont on the high-density portion is reduced by the toner charges with the progress of the developing process, a toner outermost layer potential of the high-density portion is close to Vdc. Therefore, the difference between the development contrast voltage Vcont on the high-density portion “B” and the development contrast voltage Vcont-h on the medium contrast portion “A” becomes smaller, so that the wraparound electric field disappears. As a result, the toner transferred from the developing sleeve to the medium contrast portion is prevented from transferring toward the high-density portion. Finally, the development contrast voltage on each of the medium contrast portion and the high-density portion is filled with (i.e., eliminated by) the toner charges to complete the developing process.
However, as shown in FIG. 9, when the toner charges become a state in which the development contrast voltage on the high-density portion relative to the latent image potential thereof cannot be filled (eliminated) before the image area passes through a development region, the image area passes through the development region without disappearance of the wraparound electric field. Therefore, a sufficient toner amount is not transferred to a latent image of the medium contrast portion, thereby generating the “blank area” in the interface portion “C” located between the medium contrast portion “A” and the high-density portion “B”. In FIG. 9, black portions of each of the latent image potentials correspond to a toner outermost layer potential. That is, a toner outermost layer potential on the medium contrast portion “A” is expressed by Vdc and a toner outermost layer potential on the high-density portion “B” is expressed by Vs. Therefore, in the high-density portion “B”, a potential difference (Vs−Vl) filled by developing the toner become extremely smaller than the development contrast voltage Vcont.
Note that a state in which the development contrast voltage cannot be filled with (eliminated by) the toner charges is expressed as “charging failure”.
Next, the reason why the “blank area” is not generated in a conventional organic photoreceptor but generated in the a-Si photosensitive member will be described.
The a-Si photosensitive member has a material characteristic in which a relative permittivity is three times that of the organic photoreceptor ((relative permittivity of a-Si photosensitive member): approximately 10, (relative permittivity of organic photoreceptor): approximately 3.3). When the a-Si photosensitive member has a photoconductive layer thickness equal to that of the organic photoreceptor, the a-Si photosensitive member has a capacitance three times that of the organic photoreceptor.
As a result, a toner charge amount necessary to satisfy the development contrast voltages Vcont and Vcont-h for the a-Si photosensitive member is approximately three times that for the organic photoreceptor. For example, when development is to be performed with a charged toner amount equal to that for the conventional organic photoreceptor, it is necessary that a toner amount for the a-Si photosensitive member be approximately three times that for the organic photoreceptor.
A toner amount which is present in a developing nip is limited, so the toner amount necessary to sufficiently satisfy the development contrast voltage Vcont for the a-Si photosensitive member cannot be developed under the circumstances. In particular, in order to fill the development contrast voltage Vcont on the high-density portion whose potential difference is large, an extremely large charge amount is necessary. However, because the toner amount which is present in the developing nip and can be developed is limited, the development contrast voltage Vcont cannot be filled for the a-Si photosensitive member.
As a result, the “charging failure” occurs, so that the image defect called the “blank area” generates.
Of course, even in the case of the organic photoreceptor, a high-resolution image is obtained by thin film processing for reducing the thickness of the photoconductive layer. In the organic photoreceptor, charges optically induced from the charge generation layer are diffused, so it is necessary to reduce the thickness of the photoconductive layer to improve the resolution. In order to obtain the same resolution as that in the case of the a-Si photosensitive member, it is only necessary that the thickness of the photoconductive layer of the organic photoreceptor be equal to or smaller than 17 μm as described above. When the thickness of the photoconductive layer is converted into a capacitance thereof, a lower limit thereof becomes 1.7×10−6 (F/m2). Therefore, as in the case of the a-Si photosensitive member, a high-capacitance organic photoreceptor whose capacitance is made equal to or larger than 1.7×10−6 (F/m2) by thin film processing has a problem in which the “charging failure” occurs, so that the image defect called the “blank area” generates.